There is considerable need to accurately measure the thicknesses of thin films, particularly in the semiconductor manufacturing industry. One method for making such measurements is an X-ray reflectometry technique (“XRR”) which relies on measuring the interference patterns of X-rays scattered from a thin film sample. With XRR the reflectivity of a sample is measured at X-ray wavelengths over a range of angles. These angles typically range from zero degrees, or grazing incidence along the surface of the sample, to a few degrees. From the X-ray interference pattern, properties of the sample such as material composition and thickness can be inferred.
In a recent development, simultaneous measurements of the sample reflectivity over a range of angles are accomplished by illuminating the sample with a focused beam and then detecting the reflected X-rays with a position sensitive detector such as a photodiode array.
XRR has several advantages over techniques using visible light. One such advantage is that XRR makes it possible to measure the thickness of ultra-thin films whose thicknesses are on the order of 30 angstroms or less. Visible light is not suitable for the study of such ultra-thin films using interference patterns because of its wavelength. However, an XRR system may preferably use radiation at wavelengths of about 1.5 angstroms, which radiation creates suitable interference patterns even when probing such ultra-thin films. In addition, XRR may suitably be used where the film is composed of a material that is opaque to light, such as a metal or metal compound. Finally, XRR may suitably be used to measure the density and thickness of films composed of materials that have a low dielectric constant and a correspondingly low index of refraction, such as certain polymers, carbon fluoride compounds, and aerogels.
A preferred XRR technique is described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, which is hereby incorporated by reference in its entirety. FIG. 1 illustrates this preferred technique.
Referring to FIG. 1, the preferred X-ray scattering system includes an X-ray source 31 producing an X-ray bundle 33 that comprises a plurality of X-rays shown as 35a, 35b, and 35c. An X-ray reflector 37 is placed in the path of the X-ray bundle 33. The reflector 37 directs the X-ray bundle 33 onto a test sample 39 held in a fixed position by a stage 45, and typically including a thin film layer 41 disposed on a substrate 43. Accordingly, a plurality of reflected X-rays, 57a, 57b, and 57c concurrently illuminate the thin film layer 41 of the test sample 39 at different angles of incidence.
The X-ray reflector 37 is preferably a monochromator. The diffraction of the incident bundle of X-rays 33 within the single-crystal monochromator allows only a narrow band of the incident wavelength spectrum to reach the sample 39, such that the Brag condition is satisfied for this narrow band. As a result, the plurality of X-rays 57a, 57b, and 57c, which are directed onto the test sample 39, are also monochromatic. A detector 47 is positioned to sense X-rays reflected from the test sample 39 and to produce signals corresponding to the intensities and angles of incidence of the sensed X-rays. FIG. 2 depicts an example of a graph of data from the detector 47 showing a normalized measure of the reflectivity of the sample as a function of the angle of incidence to the surface of the sample 39. A processor is connected to the detector to receive signals produced by the detector in order to determine various properties of the structure of the thin film layer, including thickness, density and smoothness.
In order to maximize the accuracy of the X-ray measurements, it is necessary to precisely calibrate and align the XRR system. The present invention relates to techniques for doing this.